I just activated my wife's 4S. Everything went smoothly except that iMessage does not always work correctly. She can send an iMessage to me, both at my iPhone number and e-mail address. However, I cannot send her one at her iPhone number. I _am_ able to use her e-mail address, though.

So the only problem seems to be that she can't receive an iMessage at her phone number. I checked the Messages settings to make sure iMessage is enabled. Her phone number shows up under "Receive At."

Does it just take time for iMessage to be activated? Mine has been active for a while (since Beta) so I can't compare my settings with hers.

Check the country chosen under:
Settings -> Messages -> Receive at -> click "AppleID: xxx" -> Change location.
This should be set to the correct country so that you're phone can use the correct country-code as prefix for the phone number, even if the number is not written with country code in your phonebook.

Could it be your wife entered your number to her phonebook as "+1xxxxx" and you've only entered hers as "xxxxx"?

Check the country chosen under:
Settings -> Messages -> Receive at -> click "AppleID: xxx" -> Change location.
This should be set to the correct country so that you're phone can use the correct country-code as prefix for the phone number, even if the number is not written with country code in your phonebook.

Could it be your wife entered your number to her phonebook as "+1xxxxx" and you've only entered hers as "xxxxx"?

Thanks for the reply.

Both our phones have the country code set to United States. Both our phone numbers start with +1 (206) xxx-xxxx.

Interestingly, iMessage works for her phone number now. They must have taken some time to enable it on her account...